This application is based on applications Nos. 2001-195878, 2001-224634, and 2001-257608 filed in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a photoelectric device using numerous crystalline semiconductor particles. This photoelectric conversion device is utilized suitably in solar cells.
The present invention also relates to a glass composition for coating silicon that is used for protecting a part of or the entire surface of silicon or for insulation between electrodes.
The present invention also relates to an insulating coating that is formed in a silicon semiconductor device and in contact with the silicon.
2. Description of the Related Art
(A) Advent of a next-generation, low-cost solar cell that allows the amount of the raw material, silicon, to be small has been eagerly awaited.
Conventional photoelectric devices in which crystalline semiconductor particles are used are shown in FIGS. 4 to 6.
FIG. 4 illustrates a structure disclosed in Japanese Unexamined Patent Publication (Kokai) No. Showa 61-124179. There is disclosed a photoelectric conversion device in which a first aluminum foil 10 is formed with apertures into which silicon balls 2 each having a n-type surface layer 9 formed on the surface of a p-type ball are inserted. The portions of the n-type surface layers 9 that have penetrated the back surface of the first aluminum foil 10 are removed, and an oxide layer 3 is formed on the back surface of the first aluminum foil 10. Portions of the oxide layer 3 that cover the silicon balls are removed, and then a second aluminum foil 8 is formed so as to join to the silicon balls 2.
FIG. 5 illustrates a structure disclosed in Japanese Patent Publication No. 2641800. There is disclosed a photoelectric conversion device in which a low melting-point metal layer 11 such as a tin layer is formed on a substrate 1. Crystalline semiconductor particles 2 of first conductivity-type are deposited on the low melting-point metal layer 11, and an amorphous semiconductor layer 7 of second conductivity-type is formed on the crystalline semiconductor particles 2 with an insulating layer 3 interposed between the low melting-point metal layer 11 and the amorphous semiconductor layer 7.
FIG. 6 illustrates a structure disclosed in Japanese Examined Patent Publication No. H08-34177. There is disclosed a method in which a high melting-point metal layer 12, a low melting-point layer 11 and fine crystalline semiconductor grains 13 are successively deposited on a substrate 1, and the fine crystalline semiconductor grains 13 are melted, saturated, and gradually cooled so that the semiconductor is grown by liquid-phase epitaxial growth, thereby forming the fine crystalline semiconductor grains 13 into a polycrystalline thin film. Incidentally, in FIG. 6, the numeral 14 denotes a polycrystalline or amorphous semiconductor layer of the opposite conductivity type, and the numeral 6 denotes a transparent conductive film.
In the photoelectric conversion device shown in FIG. 4, however, since the first aluminum foil 10 is formed with apertures into which the silicon balls 2 are pressed and inserted so as to join the n-type layers 9 of the silicon balls 2 and the aluminum foil together, the silicon balls 2 are required to have a uniform diameter. The manufacturing cost is therefore high. Also, since the temperature used for joining is lower than 577xc2x0 C., which is the eutectic temperature of aluminum and silicon, the joining tends to be unstable.
In the photoelectric conversion device shown in FIG. 5, since the insulator 3 is formed after the crystalline semiconductor particles have been fixed on the low melting-point metal layer 11, the insulator 3 is formed not only on the low melting-point metal layer 11 but also on the crystalline semiconductor particles 2. Therefore, the insulator 3 on the crystalline semiconductor particles 2 needs to be removed before the amorphous semiconductor layer 7 is formed, which causes the number of processes to increase. Since the thickness of the amorphous semiconductor layer 7 needs to be small taking the great light absorption thereof into account. When the thickness of the amorphous semiconductor layer 7 is small, the tolerance to defects also becomes small necessitating stricter management of the cleaning process and the production environment. As a result, the manufacturing cost is high.
In the photoelectric conversion device shown in FIG. 6, since the low melting-point metal layer 11 is mixed into the first conductivity-type liquid-phase epitaxial polycrystalline layer 13, the performance of the solar cell is degraded. And due to the absence of insulator, current leakage occurs between the upper electrode 6 and the lower electrode 12.
In addition, it has been known that when conventional glass compositions are employed for the insulator in conventional photoelectric conversion devices, bubbling occurs due to its reaction to the crystalline semiconductor particles, and microcracks are generated during reliability tests.
It is a primary object of the present invention to provide a photoelectric conversion device with high conversion efficiency that can be manufactured at low cost.
(B) In today""s age of intense information, information and communications technologies have rapidly been developing. Along with this trend, the demand for silicon semiconductor devices for use in MPUs and memories has been sharply on the rise. In addition, with the increasing consciousness for the environment, applications of silicon semiconductor devices other than information and communications equipment, such as solar cells, have been increasing fast.
In order to prevent errors and secure long-time reliability, the silicon is covered with an insulating coating in such semiconductor devices. By covering the silicon with an insulating coating, the silicon is protected from water and dust and insulation is provided between the electrodes.
For insulating coatings used for protecting silicon in optical semiconductor devices such as optical sensors and solar cells or insulating coatings for insulation between the electrodes in such optical semiconductor devices, transparency is required in addition to the insulation property and sealing property.
For the purpose of silicon protection, organic resin is employed for protection and sealing in applications in which the demand for reliability is relatively low. In applications for which high reliability is required, the insulating coating needs to be formed by using glass.
Generally, the insulating coatings made from glass are formed by covering silicon with glass paste, which is obtained by mixing particulate glass, organic binder, and solvent together, by a known printing method, a dispensing process, dipping or spin-coating, and thereafter performing a heat treatment to soften and fluidize the glass. The glass which has been conventionally used for forming insulating coatings to cover silicon is low softening-point glass composed mainly of PbO so that influence on the semiconductor device by heat is minimized.
However, since the PbO content needs to be large in order to lower the softening point and glass transition point, PbO-based low softening-point glass has a thermal expansion coefficient as high as 80xc3x9710xe2x88x927/xc2x0 C. at temperatures 40 to 400xc2x0 C. When PbO-based low softening-point glass with such a high thermal expansion coefficient is employed for protection and insulation in silicon semiconductor devices with thermal expansion coefficients as low as 30xc3x9710xe2x88x927/xc2x0 C. to 45xc3x9710xe2x88x927/xc2x0 C., especially in large scale silicon semiconductor devices or solar cells with greater areas, due to thermal stress accompanying the ON/OFF switching of the semiconductor device and changes in the environment of use, cracks are generated in the glass and silicon and peeling occurs at the interfaces.
In order to lower the thermal expansion coefficient of the PbO-based low softening-point glass, adding a filler with a low thermal expansion coefficient is a general practice. However, this measure causes the insulating coating to have turbidity and loose transparency.
On the other hand, when the PbO content is reduced, although the thermal expansion coefficient is lowered by which the inconvenience due to the thermal stress mentioned above can be avoided, the softening point and glass transition point rise in most cases. This increases damage by heat to the silicon semiconductor devices.
In addition, using PbO is very unfavorable considering the adverse effects on the environment. The trend toward Pb-free manufacturing is accelerating in all industry fields. Likewise, the demand for Pb-free insulating coatings for silicon is sharply on the rise.
As discussed so far, there has been a surge in demand for lead-free glass powder with a low glass transition point and a low thermal expansion coefficient. One example of such glass is the glass disclosed in Japanese Unexamined Patent Publication No. H09-278482, which is composed mainly of B2O3xe2x80x94ZnO.
However, when an insulating coating is formed by using the B2O3xe2x80x94ZnO-based glass disclosed in the Japanese Unexamined Patent Publication No. H09-278482 above, the following problems arise: the silicon and glass react to each other making the use of the silicon semiconductor device impossible; the resultant insulating coating fails to obtain adequate sealing property because of great defects accompanying generation of bubbles; the insulating coating fails to obtain transparency.
Furthermore, due to crystallization of the B2O3xe2x80x94ZnO-based glass during the heat treatment, the insulating coating cannot be formed with transparency.
The present inventors discovered that, by lowering the thermal expansion coefficient and glass transition point of B2O3xe2x80x94ZnO-based glass, and by adding SiO2 thereto, it is possible to suppress the reaction between glass and silicon, prevent generation of bubbles, and prevent crystallization at the same time. The present invention has been thus accomplished.
It is an object of the present invention to provide a glass composition for coating silicon that is capable of forming a transparent insulating coating with long time reliability, which has a low glass transition point and a low thermal expansion coefficient, and does not contain lead nor react to silicon, and is not crystallized at the desired temperature range.
It is another object of this invention to provide a transparent insulating coating in contact with silicon with a low glass transition point and a low thermal expansion coefficient, which does not contain lead and not react to silicon and is not crystallized at the desired temperature range, and has long time reliability.
(A) A photoelectric conversion device according to the present invention comprises: a substrate serving as an electrode; numerous crystalline semiconductor particles containing a first conductivity-type impurity deposited on the substrate to join thereto; an insulator provided among the crystalline semiconductor particles; and a semiconductor layer containing an impurity of the opposite conductivity-type to which another electrode is connected, which semiconductor layer being provided over the crystalline semiconductor particles, wherein the crystalline semiconductor particles comprise silicon, and the insulator comprises a glass material which contains at least 1 wt % and at most 20 wt % tin oxide.
A photoelectric conversion device according to the present invention comprises: a substrate serving as an electrode; numerous crystalline semiconductor particles containing a first conductivity-type impurity deposited on the substrate to join thereto; an insulator provided among the crystalline semiconductor particles; and a semiconductor layer containing an impurity of the opposite conductivity-type to which another electrode is connected, which semiconductor layer being provided over the crystalline semiconductor particles, wherein the crystalline semiconductor particles comprise silicon, and the insulator comprises a glass composition which contains 4.2 to 20 wt % tin oxide.
According to these photoelectric conversion devices, the insulator comprising the glass material that contains the above-stated amount of tin oxide fills spaces among the crystalline semiconductor particles and covers the whole exposed surface of the substrate without causing defects.
Accordingly, occurrence of cracking in the insulator and crystalline semiconductor particles is prevented, and defects such as bubbling and abnormal deposition can be prevented.
This allows crystalline semiconductor particles to be produced with lower grain size precision, the resultant photoelectric conversion device therefore yields a larger manufacturing margin permitting manufacturing thereof at lower manufacturing cost as compared with conventional photoelectric conversion devices.
Moreover, the presence of the insulator ensures separation of the positive electrode from the negative electrode. By employing a glass material containing tin oxide for the insulator, the molten glass and the silicon of the crystalline semiconductor particles are prevented from excessively reacting to each other. Low cost manufacturing is therefore realized.
Accordingly, it is possible to form a good insulator with stable reliability and provide a photoelectric conversion device with high reliability.
(B) A glass composition for coating silicon according to the present invention is substantially free of PbO, and contains B2O3, ZnO, and SnO2, and has a thermal expansion coefficient of 80xc3x9710xe2x88x927/xc2x0 C. or less at temperatures from 40 to 400xc2x0 C. and a glass transition point of 550xc2x0 C. or below.
An insulating coating in contact with silicon according to the present invention comprises a glass composition which is substantially free of PbO, and contains B2O3, ZnO, and SnO2, and has a thermal expansion coefficient of 80xc3x9710xe2x88x927/xc2x0 C. or less at temperatures from 40 to 400xc2x0 C. and a glass transition point of 550xc2x0 C. or below.
A method of forming an insulating coating according to the present invention comprises the steps of: covering a surface of silicon with a glass powder which is substantially free of PbO, and contains B2O3, ZnO, and SnO2, and has a thermal expansion coefficient of 80xc3x9710xe2x88x927/xc2x0 C. or less at temperatures from 40 to 400xc2x0 C. and a glass transition point of 550xc2x0 C. or below; and performing a heat treatment at a temperature of 600xc2x0 C. or below to soften and fluidize the glass powder, whereby forming an insulating coating on the silicon.
A photoelectric conversion device according to the present invention comprises silicon therein which is partly or wholly coated with an insulating coating comprising a glass composition which is substantially free of PbO, and contains B2O3, ZnO, and SnO2, and has a thermal, expansion coefficient of 80xc3x9710xe2x88x927/xc2x0 C. or less at temperatures of 40 to 400xc2x0 C. and a glass transition point of 550xc2x0 C. or below.
Since the insulating coatings applied on the surface of the silicon described above have a low glass transition points and low thermal expansion coefficients, and do not react to the silicon, they cause little bubbling and are not crystallized at the desired temperature range. Accordingly, they have transparency as well as long time reliability.
The structural details for achieving the objects of this invention are now described referring to the appended drawings.